When a fan is driven by an electric motor, what results is a combination of the properties of the fan and the properties of the electric motor.
A variety of fan designs exist, e.g. radial fans, transverse-flow blowers, axial fans, and diagonal fans. Radial fans are divided into radial fans having backward-curved blades, and radial fans having forward-curved blades. There are likewise further sub-types in the case of the other designs.
The properties of a fan result from the so-called fan characteristic curve, which indicates the quantity of air per hour (m3/h) delivered by the fan at a particular static pressure, and from the motor characteristic curve, which indicates how much power the motor needs in order to deliver a specific quantity of air per hour.
The power requirement is further determined by the operating conditions of the fan. For example, when a fan is blowing air from outside into a room in which all the doors and windows are closed, the fan is operating at maximum static pressure. “Free outlet” blowing, conversely, means that the fan is located unrestrictedly in a space, and that no physical separation, and also no pressure difference, exists between its intake side and delivery side. This therefore means that a free outlet fan has a different power requirement than a fan that is delivering into a closed space.
An examination of the curve for a fan arrangement's power consumption plotted against generated volumetric air flow rate reveals that this power is highly dependent on the working point that is set, or on the pressure buildup in the fan. In the case of a radial fan, for example, maximum power is usually reached with free outlet, i.e. at a pressure elevation Δpf=0, whereas for an axial fan, it is reached at a maximum pressure elevation Δpf=maximum.
Radial fans are normally used at a higher static pressure. When they work without static pressure, i.e. in free-outlet fashion, they are being operated at their power limit, i.e. a radial fan must be designed for this operating point even though in practice it occurs seldom and in rather arbitrary fashion. This limits the power of such a fan under other operating conditions. Analogous considerations apply to other fan types.